Over the past decade, integration of cameras with mobile devices has become increasingly common. The desired thickness for tablets and cell phones is typically less than 1 centimeter (cm) and as thin as about 5 millimeters (mm). Since the ideal camera integrated into such devices images normal to the device surface, camera thickness is subject to stringent constraints and is, in fact, a critical metric. That is, very low profile optics are required.
Market pressures dictate that a mobile-device camera provides a large field-of-view and high resolution. These requirements are, unfortunately, at odds with one another. In particular, a large field-of-view produces large off-axis aberrations that must be corrected. And to achieve high resolution, a minimal-size entrance pupil is required to achieve a particular angular resolution. As the number of pixels grows, more optical elements are required to correct these aberrations, which both become larger laterally because the entrance pupil size increases. Furthermore, the axial length of the camera increases to accommodate the increasing number of elements.
Mobile-phone cameras are purely refractive and have an entrance pupil of 2 mm or less in diameter. This limits the angular resolution to 0.015 degrees or greater, so that, for example, for a 70×42 degree field-of-view camera, there are at best 14 million resolvable pixels if the diffraction limit is achieved over the entire field.
To improve this performance, a wide entrance pupil is needed, which requires wider optics as well as a longer optical-train length.